Heat sealable packaging film comprising propylene polymer substrate and olefin surface layer blend of polybutene-1 and an ethylene or a propylene copolymer

ABSTRACT

Multiple-layer, heat-sealable films are described comprising a substrate layer of polypropylene homopolymer or copolymer, or blends thereof with compatible polyolefins; and at least one heat-sealable surface layer of a blend poly(1-butene), and a copolymer of ethylene or propylene and a higher olefin. The films are prepared by extrusion coating or coextrusion and can be oriented.

This is a division of copending application Ser. No. 082,358, filed Oct.5, 1979, now U.S. Pat. No. 4,275,120.

SUMMARY OF THE INVENTION

This invention relates to multi-layer films having a wide range of heatsealing temperatures. The films comprise a core or substrate ofpropylene homopolymer, a copolymer of propylene and another olefin or ablend of the foregoing with up to 40% by weight of a differentcompatible polyolefin, and a surface layer on one or both sides of thesubstrate. The different compatible polyolefin is preferably the samepolyolefin blend which is used for the surface layer. The heat-sealablesurface layer which is present on either one or both sides of thesubstrate is a blend of poly(1-butene) and a copolymer of ethylene orpropylene and a higher olefin.

The films of this invention can be produced by coextrusion of thesurface layer material and the substrate material followed by biaxialorientation. Alternatively, the substrate can be extruded alone,stretched in the machine direction, then extrusion coated with thesurface layer material and finally stretched perpendicular to themachine direction.

DETAILED DESCRIPTION OF THE INVENTION

Oriented polypropylene film has become a useful and widely usedpackaging film because of its good moisture barrier properties,stiffness, high-strength, and good optical properties. For widestutility, it is desired that the films be capable of being heat sealedover a broad range of temperatures. One means to achieveheat-sealability has been the application of a heat-sealable coating tothe surface. A separate coating operation following orientation,however, is quite costly.

An alternate and less costly method of achieving heat-sealability is thecoextrusion of a surface layer on one or both surfaces of a substrate.The surface layer is selected so it melts at a lower temperature thanthe underlying substrate. Coextruded oriented films have been preparedin which the oriented polypropylene substrate or core is encased inskins comprised of ethylene/propylene copolymer, butene-1 homopolymer orbutene-1/ethylene copolymer, for instance. Such films are described inU.S. Pat. No. 4,125,662.

Many of the prior art films exhibit high seal strengths but because ofthe relatively high melting point of the surface layer, the temperaturerange over which usable heat seals are formed is narrow. A narrowsealing range severely limits the utility of a packaging film. It istherefore one of the objects of this invention to provide multi-layerfilms which can be sealed over a wide range of temperatures and whichhave other desirable properties.

Another object of this invention is the provision of multi-layeredsealable films which can be prepared without separate coatingoperations.

Still another object of this invention is the provision of films whichhave one layer which is heat-sealable over a wide range to provide ahigh-strength seal, over a substrate layer which is heat-sealable toitself to provide a weaker, but useful seal.

The films of this invention comprises a substrate which ispolypropylene, polypropylene copolymer or a blend of polypropylenehomopolymer or copolymer and up to 40 weight percent of a compatiblepolyolefin or polyolefin blend. The compatible polyolefin isadvantageously the same blend as in the heat-sealable surface layer.

Copolymers of polypropylene useful as the substrate generally comprisespropylene and another olefin comonomer such as ethylene, butene orhigher olefins. Typically up to about 20 mole percent of the comonomeris used. Where the comonomer is ethylene, preferably up to 10 molepercent of the comonomer is present in the copolymer.

Polyolefin compositions which are suitable as the heat-sealable surfacelayer in accordance with this invention are:

(1) a blend of 5-19 weight percent of poly(1-butene), and 95-81 weightpercent of a copolymer of propylene (80-95 mole percent) and 1-butene(20-5 mole percent);

(2) a blend of 10-90 weight percent of poly(1-butene), and 90-10 weightpercent of a copolymer of ethylene (2-49 mole percent) and a higherolefin having 4 or more carbon atoms (98-51 mole percent);

(3) a blend of 10-90 weight percent poly(1-butene), and 90-10 weightpercent of a copolymer of ethylene (10-97 mole percent) and propylene(90-3 mole percent); or

(4) a blend of 90-10 weight percent of poly(1-butene), and 10-90 weightpercent of a copolymer of propylene (2-79 mole percent) and 1-butene(98-21 mole percent).

Methods of extrusion coating and coextrusion are well known in the artand are suitable for the production of the films in this invention.Advantageously, when extrusion coating is used, the substrate materialis extruded alone, stretched in the machine direction, extrusion coatedwith the surface material and then stretched perpendicularly to themachine direction. When coextrusion is utilized, the film obtained bycoextrusion of the substrate material and the surface material isbiaxially oriented. In either case, each surface layer constitutes 1-10%by weight of the total film.

For many applications two-layer films comprising a relatively highmelting substrate layer and an easily sealable surface layer areparticularly desirable. For example, where the film is used in packagingmachinery such as devices known in the art as "horizontal form and fill"machines it is not only unnecessary but disadvantageous to have a filmwith two low melting sealable surfaces. This is because the surface notintended for sealing necessarily comes into contact with heated areas,for example heated platens, and if the surface layer not intended forprimary sealing melts at too low a temperature sticking may occurresulting in machine malfunction. Sticking and its consequences isreduced with the two-layer films of this invention by virtue of thehigher melting point of the substrate layer which can be selected foroptimum properties.

In certain specific embodiments of this invention, the substrate filmcomprises a mixture of a homopolymer of polypropylene or a copolymer ofpropylene, and a blend compatible polyolefins. The compatible polyolefinblend preferably the same or similar polyolefin blend used for thesurface layer. The result is a film in which the adhesion between thesubstrate layer and the surface layer or layers is excellent. Anadditional advantage of such a film having only two layers is that thesurfaces of the substrate layer exhibit improved sealability to eachother as well as to the surface layer. The heat sealability of thesubstrate layer surfaces to each other is of particular advantage in theproduction of bags with heat-sealed seams. While the seams forming thebag, as such, are made by heat sealing a surface layer to a surfacelayer, it is often desired that the substrate surface be sealable toitself. For example, this is desirable in the area of the "flaps" wherea bag is made with a seal at the top, at the bottom and along the back.It is considered undesirable that the "flap" resulting from the seal atthe back of the bag protrude from the surface of the bag. With thetwo-layer films of this invention containing a proportion of the surfacelayer polymer in the substrate layer it is possible to obtain a usefuldegree of sealability of the substrate layer to itself where the flapmeets the top and bottom of the bag. The strength of this seal betweensubstrate surfaces is not nearly as strong as the strength of the sealbetween the surface layers but is sufficient to adhere the flap to thetop and bottom edges of the bag.

The invention illustrated by the following Examples.

EXAMPLE 1

A surface layer comprising a blend of 15 weight percent ofpoly(1-butene) and 85 weight percent of a copolymer of propylene (90mole percent) and 1-butene (10 mole percent) is coextruded withpolypropylene homopolymer to form a three layer sheet having apolypropylene core and polyolefin blend layer as its outer surfaces. Thethree-layer film is solidified, heated to a temperature of 225°-295° F.,stretched 5X in the machine direction, heated to a temperature of300°-330° F., and stretched 8X in the transverse direction. Thethickness of each surface layer is 0.025 mil and that of the total filmis 1.0 mil.

EXAMPLES 2-4

Example 1 is repeated except that the following blends are substitutedfor the blend used in Example 1.

    ______________________________________                                        EXAMPLE                                                                       ______________________________________                                        2        A blend of 20 weight percent of                                               poly(1-butene), and 80 weight percent                                         of a copolymer of propylene (75 mole percent) and                             1-butene (95 mole percent).                                          3        A blend of 30 weight percent of poly(1-butene)                                and 70 weight percent of a copolymer of                                       ethylene (90 mole percent) and propylene                                      (10 mole percent).                                                   4        A blend of 30 weight percent of poly(1-                                       butene) and 70 weight percent of                                              a copolymer of propylene (10 mole percent)                                    and 1-butene (90 mole percent).                                      ______________________________________                                    

EXAMPLES 5-8

Each of the blends of Examples 1-4 is extrusion coated as a surfacelayer on a polypropylene homopolymer substrate. The polypropylene sheetis preformed by extrusion, is solidified, heated to a temperature of265°-295° F., stretched 6X in the MD direction, and the extrusion coatedon one surface with each of the above described surface layer blends.The coated material is then stretched transversely 8X at a temperatureof 300°-300° F. The thickness of the film is 1.25 mils and that of thesurface layer is 0.02 mil.

EXAMPLES 9-12

The procedure of Examples 5-8 is followed except that the substratesheet is a mixture of 80% polypropylene and 20% of the surface layerblend and the substrate sheet is heated to a temperature of 225°-295° F.prior to machine direction orientation.

I claim:
 1. A multiple-layer, heat-sealable film comprising;(a) asubstrate layer consisting essentially of a homopolymer of polypropyleneor a copolymer of polypropylene which is blended with up to 40 weight %of a different compatible polyolefin; and (b) on one of the surfaces ofthe substrate layer, a surface layer comprising a blend of 10-90 wt. %poly(1-butene), and 90-10 wt. % of a copolymer of ethylene (15-96 wt. %)and propylene (4-85 wt. %); in which said different compatiblepolyolefin in the substrate corresponds to the surface layer blend. 2.The multiple-layer, heat-sealable film of claim 1 in which the substrateis polypropylene homopolymer blended with 1-40% by weight of the surfacelayer blend.
 3. The multiple-layer, heat-sealable film of claim 1 inwhich the substrate is polypropylene copolymer comprising propylene andup to 10 mole percent of ethylene blended with 1-40% by weight of thesurface layer blend.
 4. The multiple-layer, heat-sealable film of claim1 in which the substrate is polypropylene copolymer comprising propyleneand up to 20 mole percent of a higher alpha-olefin blended with 1-40% byweight of the surface layer blend.